Polycationic Ru(II) Luminophores: Syntheses, Photophysics, and Application in Electrostatically Driven Sensitization of Lanthanide Luminescence

A series of photoluminescent Ru(II) polypyridine complexes have been synthesized in a manner that varies the extent of the cationic charge. Two ligand systems (L1 and L2), based upon 2,2′-bipyridine (bipy) mono- or difunctionalized at the 5- or 5,5′-positions using N-methylimidazolium groups, were utilized. The resulting Ru(II) species therefore carried +3, +4, +6, and +8 complex moieties based on a [Ru(bipy)3]2+ core. Tetra-cationic [Ru(bipy)2(L2)][PF6]4 was characterized using XRD, revealing H-bonding interactions between two of the counteranions and the cationic unit. The ground-state features of the complexes were found to closely resemble those of the parent unfunctionalized [Ru(bipy)3]2+ complex. In contrast, the excited state properties produce a variation in emission maxima, including a bathochromic 44 nm shift of the 3MLCT band for the tetra-cationic complex; interestingly, further increases in overall charge to +6 and +8 produced a hypsochromic shift in the 3MLCT band. Supporting DFT calculations suggest that the trend in emission behavior may, in part, be due to the precise nature of the LUMO and its localization. The utility of a photoactive polycationic Ru(II) complex was then demonstrated through the sensitization of a polyanionic Yb(III) complex in free solution. The study shows that electrostatically driven ion pairing is sufficient to facilitate energy transfer between the 3MLCT donor state of the Ru(II) complex and the accepting 2F5/2 excited state of Yb(III).

The compound was suspended in CHCl3 (400 mL) and filtered.The crude material was recrystallised from hot EtOH (ca.200 mL) to obtain the title compound as a black solid (6.58 g, 20.2 mmol, 40%).
The combined organic fractions were dried over MgSO4 and the solvent removed in vacuo.The crude material was purified by column chromatography (SiO2; CH2Cl2/MeOH; 95:5) to obtain the title compound as a yellow oil (3.75 g, 22.0 mmol, 55%).

S3 Cyclic Voltammetry
Cyclic voltammetry was performed using a PalmSens4 potentiostat.Experiments were performed using HPLC grade MeCN with an analyte concentration of 1 mM at 293 K, using triply recrystallised [ n Bu4N][PF6] as the supporting electrolyte at 0.1 M concentration.A three-electrode setup was used, consisting of a platinum disc working electrode, a platinum wire counter-electrode and a silver wire pseudo-reference.Solutions were spared for 10 minutes with MeCN saturated stream of nitrogen gas.Voltammograms were referenced to the ferrocene/ferrocenium redox couple measured using the same conditions.

Figure S36 .
Figure S36.Cyclic voltammograms for the family of ruthenium complexes

Table S6 .
A description of the calculated MO contributions, excited state descriptions and their associated transitions for [Ru(bipy)2(L1)] 3+ where Bipy1 and Bipy2 are the two distinct bipyridine ligands.

Table S7 .
A description of the calculated MO contributions, excited state descriptions and their associated transitions for [Ru(bipy)2(L2)] 4+ where Bipy1 and Bipy2 are the two distinct bipyridine ligands.

Table S10
Computed values for the S0 → T1 energy gap from vertical TD-DFT calculations.